Direct and Indirect Effects of Climate on Bird Abundance Along Elevation Gradients in the Northern Appalachians
The stratification of bird species along elevational gradients is widely reported, with montane bird communities typically characterized by distinctive species occurring in relatively small and isolated populations; as such, these species are the subject of considerable interest to ecologists and conservationists. The stratification of species along elevation is largely attributed to compressed climatic zonation. Recent evidence that bird species are shifting up in elevation has fueled speculation that these species are tracking their climactic niches in response to climate change. However, there is also evidence plant communities are shifting in elevation, presenting a potential additional mechanism explaining changes observed in the bird community. Uncertainty as to the degree to which climate directly influences bird abundance versus the degree to which climate indirectly influences bird abundance via habitat composition and structure represents a key impediment to understanding the ecology of these species in montane environments. To address this question I measured species abundance, habitat characteristics, and temperature at 150 survey points located along 15 elevational transects in the Presidential Mountains of New Hampshire in the summers of 2014 and 2015. I used N-mixture models to correct for imperfect detection of species and structural equation models, incorporating abundance, habitat, temperature, and precipitation derived from a downscaled regional dataset to assign variation to the direct and indirect effects of climate upon birds. Analysis of 21 species revealed species-specific patterns on how climate exerts direct effects and indirect effects mediated by forest composition and structure on bird abundance. This work represents an important contribution to the ecological understanding of the pathways by which climate influences bird abundance. Finding that 62% of species experience both direct and indirect effects of climate, with 62% experiencing stronger direct than indirect effects, these results underscore the vulnerability of these species to climate change. With 81% of species found to experience indirect effects of climate via forests, these findings indicate great conservation value of maintaining forest habitat amidst climate change. Overall, this information will facilitate the refinement of predictive models of the abundance of montane bird species and represents an approach that will advance future investigations of climate effects in the Northern Appalachians and other systems.